The organic EL device is a self-emitting device, and has been actively studied for their brighter, superior viewability and ability to display clearer images compared with the liquid crystal device.
In 1987, C. W. Tang et al. at Eastman Kodak developed a laminated structure device using materials assigned with different roles, realizing practical applications of an organic EL device with organic materials. These researchers laminated an electron-transporting phosphor and a hole-transporting organic material, and injected the both charges into the phosphor layer to cause emission in order to obtain a high luminance of 1,000 cd/m2 or more at a voltage of 10 V or less (refer to Patent Documents 1 and 2, for example).
To date, various improvements have been made for practical applications of the organic EL device. As an example, the various roles of the laminated structure are further subdivided to provide an electroluminescent device in which an anode, a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and a cathode are successively formed on a substrate, and a light-emitting device of a bottom-emission structure (the light is emitted from the bottom) having such a configuration has been proposed to realize high efficiency and durability (refer to Non-Patent Document 1, for example).
Light-emitting devices of a top-emission structure (the light is emitted from the top) that use high work-function metals for the anode have been used lately. An advantage of the light-emitting device of a top-emission structure is the wide emitter area, which is restricted by the pixel circuit in the light-emitting device of a bottom-emission structure. In the light-emitting device of a top-emission structure, the cathode uses a semi-transparent electrode such as LiF/Al/Ag (refer to Non-Patent Document 2, for example), Ca/Mg (refer to Non-Patent Document 3, for example), and LiF/MgAg.
In such light-emitting devices, total reflection occurs at the interface between a light emitting layer and other films when the emitted light from the light emitting layer is incident on other films above certain angles. The device can thus make use of only a part of the emitted light. For improved coupling-out efficiency, there have been proposed light-emitting devices that include a high-refractive-index “capping layer” on the outer side of a semi-transparent electrode of a lower refractive index (refer to Non-Patent Documents 2 and 3, for example).
The capping layer in a light-emitting device of a top-emission structure has been shown to effectively increase current efficiency about 1.7 fold in a light-emitting device using Ir(ppy)3 as the light-emitting material, from 38 cd/A without a capping layer to 64 cd/A with a 60 nm-thick ZnSe capping layer. It has also been indicated that maximizing the transmittances of the semi-transparent electrode and the capping layer does not necessarily yield the maximum efficiency, and that the maximum coupling-out efficiency is determined by the interference effect (refer to Non-Patent Document 3, for example).
It has been proposed to use a fine metal mask for the formation of a capping layer. A problem of such metal masks, however, is that the registration accuracy decreases because of strains due to heat. Specifically, ZnSe has a high melting point of 1100° C. or more (refer to Non-Patent Document 3, for example), and cannot be vapor deposited on an accurate position with a fine mask. In fact, many inorganic materials have high vapor deposition temperatures, and are not suited for use with fine masks, and may even damage the light-emitting device itself. A capping layer formed by using inorganic materials is also not usable when sputtering is used for the deposition because sputtering damages the light-emitting device.
Tris(8-hydroxyquinoline)aluminum (hereinafter, “Alq3”) is used as a capping layer for adjusting the refractive index (refer to Non-Patent Document 2, for example). Alg3 is an organic EL material commonly used as a green-emitting material or an electron transport material, and has a weak absorption near 450 nm used for blue-emitting devices. This is problematic because it lowers the color purity of a blue-emitting device.
Another problem is that the material involves large differences in refractive indices measured in the blue, green, and red wavelength regions, preventing high coupling-out efficiency from being obtained in all of the blue-, green-, and red-emitting devices at the same time.
In order to improve the device characteristics of organic EL devices, particularly to greatly improve the coupling-out efficiency, there is a need for a capping layer material having a high refractive index that involves only small differences in refractive indices measured in the blue, green, and red wavelength regions, and that excels in thin-film stability and durability.